In the increasing literature dealing with the potential applications of blockchain technology in the energy sector, one key aspect is under-estimated: the use of renewal energies to fuel the energy consumed by the blockchain technology. The vast majority of blockchain-based projects use the Proof-of-Work (POW) consensus algorithm, which paradoxically is well-known to consume a high level of electricity - how can a new solution promote green energy with transactions that are validated through a non-green process (POW protocol)? This perspective discusses this apparent contradiction by debating the extent to which the cryptographic validation of the POW-based blockchain technology (illustrated by Bitcoin here) can really fuelled with green electricity. I explain that the vast majority of the literature dealing with the ecological influence of Bitcoin mainly focus on the energetic dimension under-estimating the environmental impact. Integrating research about Life Cycle Assessments, I suggest here a conceptual framework rejecting the implicit assumption that renewable energy fuelled mining industry would make POW based operations greener. In the light of the recent ban of mining operations in China and Iran, the issue discussed in this article is timely for policymakers who need to promote the development of standards for mining operations related equipment in order to avoid potential negative environmental side effects.